The essential requirements for the extended cavity are that the laser diode, the diffraction grating, and a collimating lens all be located rigidly with respect to each other, and that the angle of the grating and the position of the lens be precisely adjustable. Our design (Fig. 1) takes advantage of the fact that these requirements can now be satisfied with convenient low-cost commercial components. A collimation tube (ThorLabs LT110P-B) holds the laser diode and the collimating lens (an aspheric optic with f=6.2 mm and NA = 0.4). The threaded tube also provides precisely adjustable focusing of the collimating lens, and it accurately locates the collimating lens axis along the laser diode axis. The cavity itself is constructed on a mirror mount, eliminating the need for the milled baseplate with separate diode, collimating lens, and grating assemblies of other designs. The diffraction grating is mounted on the front plate of the mirror mount, and the collimation tube assembly on the back plate. The beam is coupled out of the cavity in the zeroth-order reflection from the grating. We have used the Newport Ultima U100-P mirror mount because of its good mechanical stability and because its thick plates are convenient for mounting the grating and collimation tube. The minor modifications made to the mirror mount are shown in Fig. 2. Two holes are tapped in the front plate of the mount and a square section of the plate is cut away. In addition, a clearance hole and clamping screw are added to the Ultima UPA-PA1 post adapter to mount the collimation tube. The total length of the extended cavity is about 20 mm, which is long enough to reduce the linewidth below 1 MHz, while at the same time the corresponding mode spacing of ~8 GHz is large enough to give a useful continuous scan range and robust single-frequency operation.